Field of the Invention
This invention relates to fasteners for relatively soft materials which are self-locked in position by flaring expansion of the fastener shank as it is advanced in the material to be fastened.
Two desirable characteristics of fasteners are high resistance to pulling out when subjected to a tensile load and resistance to backing out by unscrewing. It is particularly difficult to obtain those characteristics when the fastener must be used in materials such as wood, plastic, sheetrock, and the like. These materials are often not as strong as the fastener, leading to stripping of the threads in the material and pulling-out of the fastener. Locking a fastener in such material also presents a problem for the same reason.
It is desirable that the fastener be a self-contained unit, alleviating the need for nuts and lock washers and therefore access to the backside of the material the fastener is connected with. A prior art approach used for fastening to wood includes a bushing with interior and exterior thread which is threaded into a hole into the wood and receives a fastener in the interior threads. Such a bushing is basically ineffective in sheetrock and the like because of that material's very low strength. Devices for fastening to sheetrock generally require drilling a hole through the material and inserting through the hole a device which expands on the opposite side to provide an increased surface to disperse the force applied to the fastener. A typical example is a toggle bolt.
It is apparent that forming threads in such materials for use with a conventional threaded fastener such as bolts or screws is not wholly satisfactory. There are several prior art devices that provide a good and reliable means of fastening such as materials, however, most, if not all, require the usage of additional parts or special preparation to function effectively.